This invention relates to a picture processing method in which a picture including half-tones is reproduced with binary outputs.
In general, a digital type copying machine or a picture input and output device such as a facsimile unit has a serious problem to be solved. That is, in the case where a video signal read at the input side is transmitted to the output side, it is desirable to improve the efficiency of transmission of the video signal, and when it is required to store the contents of the video signal, it is preferable to reduce the storage capacity as much as possible, and yet it is required that a picture reproduced with binary outputs at the output side is as similar to the original picture as possible.
In the art, a so-called "systematic dither method" is known as a method in which a picture such as a photograph including half-tones is scanned for every picture element to provide a video signal, which is processed to produce a high tone picture with binary outputs. In the systematic dither method, a picture made up of picture elements in matrix form is covered, in its entirety, with a sub-matrix in nxn arrangement, and a dither pattern which has been provided for the sub-matrix in advance is employed, so that the level of each picture element is determined as "black" or "white" with the dither values in the dither pattern as threshold values.
However, the systematic dither method is disadvantageous in the following point: When picture data obtained by sampling a picture including half-tones at a picture element density of n pel/mm are subjected to dither process, then the output picture will have a picture element density of n pel/mm. Therefore, even if the original picture is read by sampling it at a low picture element density in order to improve the efficiency of transmission as described above and the picture data are subjected to a dither process, the produced picture is low in resolution. That is, it is impossible for the systematic dither method to produce a half-tone picture which is excellent in reproducibility.